1. Field of the Invention
The present invention relates to a mobile radio communication apparatus, and in particular to a mobile radio communication apparatus for accomplishing antenna switching diversity.
2. Description of the Related Art
In mobile communication, the antenna switching diversity method is used as a method for selecting a transmission path with a minimal characteristic deterioration of incoming signals due to interference waves or fading.
The antenna switching diversity method involves installing a plurality of antennas, using each of the antennas to receive signals transmitted through a plurality of different transmission paths, receiving signals by selecting an antenna that receives the strongest receiving level among the branches of these incoming signals, and, when the receiving level of the selected antenna falls below a predetermined threshold value, receiving signals by switching to an antenna whose receiving level is higher than the threshold level.
A disadvantage of conventional devices involving the use of this antenna switching diversity method is that because antenna switching occurs only when the received signal level changes from a value that is higher than the threshold value to a value that is lower than the threshold value for an antenna in the receiving mode, the result is that when, for example, the receiving levels of two antennas fall below the threshold value in the case of switching diversity performed using two antennas, the fact that the receiving level of the antenna currently not in the receiving mode subsequently returns to a level above the threshold value fails to ensure that a change is made to this antenna from the receiving antenna whose receiving level is below the threshold value, that is, fails to produce the diversity effect. This is caused by a fact that since the threshold value is a fixed value, a received signal level stays at a value lower than the fixed threshold value when a poor signal receiving condition continues.
There is another method to overcome this deficiency. When the level of the receiving antenna has fallen below the threshold value and another antenna is selected and the level of another antenna falls below the value, the initial antenna is again selected. A problem occasionally encountered in such cases, however, is that when the receiving levels of both antennas fall below the threshold value, fast switching continuously occurs between the two antennas, and the noise generated by such switching makes it virtually impossible to receive signals.
An undesirable phenomenon that sometimes results is that when the threshold level is lowered to avoid this, it becomes impossible to fully obtain the targeted diversity effect.